This invention relates generally to cathode ray tube (CRT) faceplates and is particularly directed to reducing inner surface reflection toward a viewer in a flat faceplate such as used in a color CRT with a flat tensioned shadow mask.
A conventional CRT consists of an evacuated envelope having a neck portion, a faceplate, and a funnel portion therebetween. An electron gun disposed in the neck portion of the envelope emits energetic electrons which are directed onto the inner surface of the faceplate. Disposed on the inner surface of the faceplate are a large number of phosphor elements which glow momentarily when struck from the rear by electrons from the electron gun to produce a video image which is visible through the faceplate.
A CRT is particularly susceptible to the effects of ambient light incident upon its faceplate. Ambient light produces reflections from both the outer, or forward, surface of the CRT's faceplate as well as from its inner, or aft, surface. In the past, the curved surfaces of the CRT's faceplate have typically been roughened to a given surface texture either by pressing the glass surface, called stippling; by using a chemical process such as acid etching; or have been covered with a layer of an anti-reflection coating to reduce reflections directed back toward the viewer which degrade the video image. The pressing and acid etching approaches roughen the surfaces of the faceplate so as to reduce specular reflection from the faceplate and increase its diffuse reflection. The layers of anti-reflection coating deposited on the faceplate's surfaces possess interference properties selected so as to minimize ambient light reflected from its forward and aft surfaces toward the viewer.
One example of the use of surface coatings on a CRT's faceplate for reducing reflections can be found in U.S. Pat. No. 4,310,784 to Anthon et al, wherein it is disclosed the use of an anti-reflection coating disposed on the outer surface of the faceplate to reduce reflection and suppress the central portion of he halo surrounding an illuminated spot on the faceplate. An angle sensitive thin film interference coating which exhibits high transmittance for light emitted by the phosphor at low angles of incidence and high reflectance for light emitted by the phosphor at high angles of incidence is disposed on the inner surface of the faceplate to suppress the outer ring-like portion of the halo arising from internal reflection of light generated by the phosphor screen. U.S. Pat. No. 4,310,783 to Temple et al discloses the use of an absorbing filter disposed between the phosphor screen and faceplate of a CRT for absorbing light emitted from the phosphor screen in combination with an angle sensitive shortwave pass filter disposed between the phosphor screen and the absorbing filter for reflecting light emitted at a high angle from the phosphor screen to reduce reflections from the faceplate and for suppressing the halo effect of the illuminated phosphor dots. U.S. Pat. No. 3,209,191 to Hamilton discloses a CRT screen and ambient light filter disposed between the phosphor layer and the inner surface of the glass faceplate which is comprised of a transparent material in which are embedded a plurality of filter elements having a grid pattern which causes ambient light rays striking the surface of the filter body at acute angles other than normal to be refracted into cells formed within the grid pattern such that the light rays strike one or another of a plurality of depthwise spaced lines of opaque or light absorbing material and are absorbed thereby. The aforementioned approaches reduce light incident at angles off of normal, but not light generally normal to the surfaces of the faceplate such as would be present in a bright room and which would appear as a reflection of the room itself or of a viewer positioned in front of the CRT.
Recent work in the area of video displays has led to the development of color CRT's which employ a flat faceplate in combination with a flat tensioned shadow mask which provide improved brightness and/or contrast of the video image. Although offering enhanced video imagery, CRTs with a perfectly flat faceplate are still subject to ambient light reflection. Attempts to reduce reflection in this type of CRT have paralleled efforts in this area with respect to curved faceplate CRTs and have generally involved a roughening of the faceplate's surfaces using mechanical, e.g., sandblasting, lapping, etc., or chemical, e.g., etching, techniques. More specifically, some approaches to reducing faceplate internal reflection have involved a stepwise procedure consisting of pressing the surfaces of the faceplate during fabrication to provide a generally planar surface, grinding or lapping the faceplate until its surfaces are perfectly flat, then polishing to a mirror finish and subjecting the faceplate to an acid etching process providing the thus polished surfaces with a given texture. The polishing operation is necessary to remove the scratches and other irregularities incorporated in the glass surface during the first lapping operation. This multi-step approach provides a flat faceplate which exhibits relatively low reflectance and offers a high degree of video image acuity, but requires considerable processing time and increases the cost of flat faceplates thus produced. In addition, acid etching leads to an increase in the size of minute scratches in the glass surface resulting in reduced video image acuity and, where severe, rejection of defective faceplates.
It would be desirable from cost and process time considerations to be able to generate the low specular reflection surface mechanically, i.e., by lapping, but most conventional lapping processes use a soft cast iron lapping wheel which as indicated above is prone to scratching, making the resultant surface unusable. In addition, the grit imbeds itself into the cast iron surface, creating a very aggressive grind, which causes the final surface to be too rough. The basis for the present invention includes the discovery that lapping by rubbing a hand sized piece of glass over the piece being lapped, with a water suspension of grit between the surfaces, a very gentle lap could be produced with a roughness capable of eliminating specular reflections, but still smooth enough to allow acceptable application of the black matrix screen.
Apparently the glass surfaces are hard enough to prevent the grit from imbedding, allowing the grit to roll rather than scrape over the rubbing surfaces.
It was also found that use of this type of lapping was sufficient to eliminate the intermediate polishing step, since the scratches produced by conventional lapping were eliminated by this free abrasive process.